CO2 Electrolyzer

ABSTRACT

Various embodiments include a CO2 electrolyzer comprising: a gas space adjoining a cathode comprising a gas diffusion electrode adjoining a cathode space; an anode in an anode space; a membrane separating the cathode space from the anode space; a feed apparatus feeding reactant gas into the gas space; a mixing vessel for at least partial joint accommodation of the anolyte and the catholyte; and a connection line between the gas separating region and the gas space. The mixing vessel includes a gas separating region closed off with respect to a surrounding atmosphere. The cathode space accommodates a catholyte and the anode space accommodates an anolyte.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of International Application No. PCT/EP2018/067028 filed Jun. 26, 2018, which designates the United States of America, and claims priority to DE Application No. 10 2017 212 278.1 filed Jul. 18, 2017, the contents of which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to electrolysis. Various embodiments may include CO₂ electrolyzers and/or methods for operating a CO₂ electrolyzer.

BACKGROUND

A CO₂ electrolyzer, that is to say an electrolyzer in which at least to some extent CO₂ is introduced as reactant gas, is suitable for various products based on carbon dioxide, for example carbon monoxide or organic products which contain carbon and hydrogen. The use of an electrolyzer is particularly advantageous in this case when there is excess electrical energy in a power grid and hence chemical substances of value are intended to be produced using this excess electrical energy.

One concept for CO₂ electrolyzers is based on an aqueous electrolyte containing dissolved conductive salt, which is conducted through electrolysis cells having a separating membrane.

The cathode-side portion of the electrolyte is called the catholyte, the anode-side portion is correspondingly called the anolyte. The cathode comprises a gas diffusion electrode so that sufficiently good contact between a gas phase comprising the reactant gases and the catholyte can be brought about. The gas is expediently passed along behind the gas diffusion electrode so that a product gas is obtained at the end of the electrolysis cell without it coming into direct contact with the catholyte. A separation of the gas phase from the catholyte is therefore no longer necessary for obtaining the product.

One particularity of this electrolysis concept consists in that the pH of the aqueous catholyte must not be too low since otherwise hydrogen would be formed at the cathode. Instead, the catholyte must lie in the neutral range or be basic. In practice, this results in the case of a CO₂ electrolyzer in one molecule of CO₂ being transferred from a gas space through the gas diffusion electrode into the catholyte per electron that has flowed. This means that for the conversion of one molecule of CO₂ into the substance of value CO, that is to say carbon monoxide, for which two electrons are required, two molecules of CO₂ pass over into the electrolyte.

This leads to an accumulation of hydrogencarbonate in the electrolyte. This results in turn in the carbon dioxide stored as hydrogencarbonate ultimately being released again as CO₂ in the process. This means that the proportion of supplied carbon dioxide which is converted into a substance of value, depending on the process regime, is considerably less than 100%, which jeopardizes the economic viability of the process.

SUMMARY

The teachings of the present disclosure describe CO₂ electrolyzers and also methods for operating a CO₂ electrolyzer, in which, compared to the prior art, markedly less carbon dioxide that is introduced as reactant gas is lost in the process. As an example, some embodiments include a CO2 electrolyzer having a gas space (4) that adjoins a cathode (6) which is formed as a gas diffusion electrode (7) which in turn adjoins a cathode space (8), and having an anode (10) having an anode space (12), wherein the cathode space (8) and the anode space (12) are separated by a membrane (13), wherein the cathode space (8) is intended to accommodate a catholyte (14) and the anode space (12) is intended to accommodate an anolyte (15) and the gas space (4) has a feed apparatus (16) for the feeding of reactant gas (18), the electrolyzer (2) additionally comprises a mixing vessel (20) for at least partial joint accommodation of the anolyte (15) and the catholyte (14), characterized in that the mixing vessel (20) has a gas separating region (24) which is closed off with respect to an atmosphere (22) and a connection line (26) is provided between the gas separating region (24) and the gas space (4).

In some embodiments, a mixture of catholyte (14) and anolyte (15) that is present in the mixing vessel has a concentration C of negative charge carriers and the cumulative proportion of hydrogencarbonate ions, carbonate ions and hydroxide ions is less than 20% of the total concentration of negative charge carriers, or less than 10% of C.

In some embodiments, a feed apparatus (28) for the reactant gas (18) is provided on the mixing vessel (20) and the reactant gas (18) is fed into the gas space (4) at least partially via the mixing vessel (20).

In some embodiments, the electrolyzer (2) has a cathode (6) which is formed in the form of a gas diffusion electrode (7) and which adjoins a cathode space (8) through which in turn a catholyte (14) flows, the electrolyzer (2) additionally has an anode space (12) which adjoins the cathode space (8), is separated therefrom by a membrane (13) and in which an anode (10) is arranged, wherein an anolyte (15) flows through the anode space (12) and wherein a CO2-containing reactant gas (18) is introduced into a gas space (4) adjoining the gas diffusion electrode (7), in addition, after flowing through the cathode space (8) or the anode space (12), the catholyte (14) and the anolyte (15) are conducted into a mixing vessel (20), wherein a CO2-containing gas (23) is evolved from a mixture (21) of the liquid catholyte (14) and the liquid anolyte (15) and in turn is fed to the gas space as part of the reactant gas (18).

In some embodiments, an operating pressure prevailing in the electrolyzer (2) is less than 5 bar, less than 1 bar, or less than 0.5 bar.

In some embodiments, the reactant gas (18) is conducted through the mixing vessel (20) and is introduced together with the CO2-containing gas (23) evolved there into the gas space (20).

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the teachings herein and further features are elucidated in more detail on the basis of the following figures. These are purely exemplary embodiments which do not represent any restriction of the scope of the disclosure. In the figures:

FIG. 1 shows a schematic illustration of a CO₂ electrolyzer in which carbon dioxide-containing gas which has been separated is supplied to the gas space from a mixing vessel; and

FIG. 2 shows an electrolyzer as per FIG. 1, wherein reactant gas is conducted via the mixing vessel into the gas space of the electrolyzer.

DETAILED DESCRIPTION

Some embodiments of the teachings herein include a CO₂ electrolyzer with a gas space that adjoins a cathode which is formed as a gas diffusion electrode. The gas diffusion electrode in turn adjoins a cathode space; the electrolyzer additionally comprises an anode space and an anode. The cathode space and the anode space are arranged in a manner separated by a membrane. The cathode space is suitable for accommodating what is known as a catholyte, in contrast the anode space is intended to accommodate an anolyte. The catholyte and anolyte are hereafter referred to in general as electrolytes. There is also a feed apparatus for reactant gases at the gas space, and a mixing vessel which is suitable for jointly accommodating at least portions of the anolyte and the catholyte. In some embodiments, the mixing vessel has a gas separating region which is closed off with respect to an atmosphere and in that a connection line is provided between the gas separating region of the mixing vessel and the gas space.

The term “gas diffusion electrode” is used herein to mean an electrode at which three states of matter, namely solid, liquid and gaseous, are in contact with one another. The solid phase is in this case formed by a catalyst (preferably applied to the electrode surface), which catalyzes an electrochemical reaction between the liquid phase (generally the electrolyte) and the gaseous phase (generally the reactant gas). Here, the reactant gas is a gas which at least partially contains carbon dioxide and is at least partially converted at the gas diffusion electrode into a substance of value—the product.

The catholyte and the anolyte are—generally aqueous-based—liquids in which what are known as conductive salts are dissolved. In order to equalize concentrations of conductive salt ions in the electrolytes, these two liquid phases are at least partially conveyed into the mixing vessel. This counteracts a separation which inevitably occurs during the electrolysis process. The gas separating region is part of the mixing vessel, and it serves to ensure that gases dissolved in the anolyte and in the catholyte outgas from the liquid and preferably collect above the liquid surface of the anolyte and catholyte. This region in which the released gases collect is called the gas separating region. This gas separating region is closed off with respect to an atmosphere, that is to say with respect to the space in which the electrolyzer is set up. That is, essentially no gas (aside from leakages) can escape unhindered from the gas separating region into the atmosphere or in other words into the ambient air. Specifically mounted feed and discharge lines and also safety apparatuses such as pressure relief valves are exempt from being closed off from the atmosphere.

In some embodiments, due to the gas space being closed off with respect to the atmosphere, gas, in particular carbon dioxide, which escapes from the catholyte and the anolyte or from a mixture of these two liquid electrolytes, is separable/isolable and can be fed in particular as reactant gas back to the gas space via a connection line. That is, the reactant gas, in particular the carbon dioxide, which for construction-related reasons gets into the catholyte or anolyte via the gas diffusion electrode, is recovered and fed once more to the gas space as reactant gas. In this way, the efficiency of the electrolyzer compared to an electrolyzer of comparable construction can be increased.

In some embodiments, both the catholyte and the anolyte have, as conductive salts, only a low concentration of salts containing hydrogencarbonate ions, carbonate ions or hydroxide ions. The ions mentioned have a tendency to absorb the carbon dioxide and incorporate it in chemically bound form in the catholyte/anolyte. By means of the low proportion of these mentioned ions, which may be less than 20% of the total concentration of negative charge carriers in the anolyte and/or catholyte, or less than 10%, the absorption of carbon dioxide at the anolyte or catholyte can be reduced, which also improves the efficiency of the electrolyzer and also makes the separation in the gas separating region and the collection of carbon dioxide in the gas separating region more efficient.

In some embodiments, a feed apparatus for the reactant gas is provided on the mixing vessel and the reactant gas is fed into the gas space at least partially via the mixing vessel. This means that the reactant gas is not fed directly to the gas space, but instead it is first of all at least partially conducted via the gas separating region of the mixing vessel. In some embodiments, no extra fan is necessary from the gas separating region of the mixing vessel in order to convey the carbon dioxide separated there into the gas space. Said gas separated there is thus drawn along by the reactant gas introduced and integrated into the stream thereof.

In some embodiments, the electrolyzer has a cathode which is formed in the form of a gas diffusion electrode and which adjoins a cathode space. A catholyte in this case flows through the cathode space, wherein the cathode space is separated off with respect to an anode space by a membrane. An anolyte is in turn passed through the anode space and an anode is arranged in or at the anode space. A reactant gas containing CO₂ is additionally introduced into a gas space adjoining the gas diffusion electrode. In addition, after flowing through the anode space or the cathode space, the catholyte and the anolyte are brought into a mixing vessel, where these at least partially mix and as a result equalize the concentration thereof. A carbon dioxide-containing gas is evolved from this mixture of the liquid anolyte and catholyte and in turn is fed to the gas space as part of the reactant gas.

In some embodiments, an operating pressure of less than 5 bar, less than 1 bar and/or less than 0.5 bar, prevails in the electrolyzer. A lower operating pressure results in a lower dissolution of carbon dioxide in the electrolyte, that is to say in the catholyte/anolyte, which increases the yield of product gases.

FIG. 1 shows an electrolyzer 2, the latter having in the central region a gas space 4 which is delimited by a gas diffusion electrode 7 which here also forms the cathode 6. The boundary surface that the gas diffusion electrode 7 forms is formed with respect to a cathode space 8, wherein the cathode space 8 in turn is separated on a further side from an anode space 12 by a membrane 13. An anode 10 is arranged in or at the anode space 12. An electrolyte in liquid form flows through both the cathode space 8 and the anode space 10. The electrolyte that flows through the cathode space is referred to as the catholyte, the electrolyte that flows through the anode space is accordingly referred to as the anolyte. The electrolytes that flow out of the anode space 12 and the cathode space 8 are conducted via an electrolyte line 17, 17′ to a mixing vessel 20. In the mixing vessel 20 (at least portions of) the anolyte 15 and also (of) the catholyte 14 are mixed to form a mixture 21 which results in a concentration equalization of the ions present in the individual electrolytes. Whether the electrolytes are brought together completely or only in portions in the mixing vessel depends in principle on the current process regime and the concentration equalization that becomes necessary as a result thereof.

At the same time, a CO₂-containing gas 23 that forms from the mixture of anolyte 15 and catholyte 14 is evolved in the mixing vessel 20 above a liquid surface of the mixture 21. This CO₂-containing gas 23 originates from the gas which got into the catholyte via the gas diffusion electrode 7 and possibly also into the anolyte via the membrane 13. This gas 23 collects in a gas separating region 24 in the mixing vessel 21. This gas separating region 24 is closed off with respect to an atmosphere 22. This means that no gas emerges undesirably into the atmosphere, rather the gas 23 that is present in the gas separating region 24 is fed in a controlled manner to the feed apparatus 16 for the reactant gas 18 via a connection line 26 and is introduced into the gas space 4 as part of the reactant gas. This reactant gas 18 is catalytically converted at the gas diffusion electrode into a product, in particular a product gas 19, that may for example be carbon monoxide.

The electrolyzer 2 of FIG. 2 differs from the electrolyzer 2 of FIG. 1 in that the feed apparatus 16 of the reactant gas 18 is formed in such a way that the reactant gas 18 is fed via the mixing vessel 20 and the reactant gas 18 is also conducted via the gas separating region 24 and is conducted into the gas space 4 via a further feed line 26 which then forms part of the feed apparatus 16. The difference from the electrolyzer 2 of FIG. 1 consists in that here the stream of the reactant gas 18 is exploited in order to transport the CO₂-containing gas 23 out of the gas separating region and deliver it into the gas space 24. In FIG. 1, a fan (not illustrated) would be necessary for this, which in turn requires a certain amount of energy in order to be operated.

Hereafter, further physicochemical aspects of the teachings and of the operation of a CO₂ electrolyzer in general are elucidated. In some embodiments, a relatively large number of cells of an electrolyzer 2 may be combined within a cell stack, which is also referred to as a stack. In order to obtain an electrolyzer system, which has not been illustrated graphically here, this cell stack is combined with additional peripheral equipment for cooling, reactant feed and product removal, but also with the infrastructure for the anolyte 15 and the catholyte 14. There are many different possibilities for connecting up the peripheral equipment (not illustrated here).

A complete separation of anolyte and catholyte can in principle be realized only with high technical complexity, since transport of ions through the membrane must be possible. This transport is ion selective. In addition to the ions, water can likewise get through the membrane, which leads to a concentration or a dilution of the electrolyte streams. Accordingly, separate circuits for anolyte and catholyte within the peripheral equipment would result in their compositions diverging, which would lead to undesired effects, such as for example a higher electrolysis voltage or an excessively high salt concentration with precipitation of solids. For this reason, anolyte 15 and catholyte 14 are at least partially mixed in the mixing vessel 20. As a result of this, the concentrations of the conductive salt after passage through the anode space 12/cathode space 8 are brought back in line. Here, FIGS. 1 and 2 show complete mixing of anolyte 15 and catholyte 14, but this does not necessarily have to be the case. In principle, it may also suffice to mix smaller amounts or portions of the anolyte 15 and of the catholyte 14 with one another per pass in order to ensure concentration equalization.

In the case of the structure shown in FIG. 1 and FIG. 2, there is a location at which accumulated carbon dioxide can escape from the electrolyte, that is to say the catholyte 15 or the anolyte 14, and this is specifically the separating vessel 29 at which anode gas 30 can be discharged. The gas separated there may in the case of an appropriate process regime also be CO₂-rich and can likewise be fed back to the gas space 4, which would, however, necessitate separation of oxidizing gas which is likewise present in the anode gas, generally oxygen. This approach is not illustrated at this point.

In order to achieve a high conductivity of the aqueous electrolyte, the latter contains a dissolved salt, which is also referred to as conductive salt. Since the ionic species that form during the electrolysis perform charge transport through the fluid, this conductive salt in the electrolyte, be it anolyte 15 or catholyte 14, is important. Useful conductive salts are in principle all substances which form ions in dissolved form. Strong electrolytes may be used here, since these dissociate practically completely and thus a maximum amount of ionic species forms for a given amount of conductive salt used. Typical candidates for conductive salts are the salts of the alkali metals and of the alkaline earth metals, of the mineral acids, for example potassium sulfate, calcium chloride or sodium nitrate. However, salts of phosphoric acid and of carbonic acid can also be used. Mixtures of different salts are also particularly advantageous, since in this way higher solubilities and, as a consequence, higher conductivities are possible. A conductive salt could therefore, for example, consist of a mixture of potassium hydrogencarbonate and potassium sulfate.

Conductive salts that contain CO₂/can chemically bind CO₂ can lead to chemically bound carbon dioxide getting into the anode space 12 and being released again there due to a pH change brought about by the anode reaction. Carbonates, hydrogencarbonates, and hydroxides are involved here. Carbonates and hydroxides can react with CO₂ to form hydrogencarbonate. A similar, likewise undesirable, transport effect can be brought about as a result of physically dissolved carbon dioxide, with this occurring in particular at high operating pressures during the electrolysis. If in practice physically dissolved CO₂ is brought into the anode space with the anolyte 15, this inevitably seeks to convert into the gas phase. The reason for this is the anode gas formed, which lowers the CO₂ partial pressure and therefore leads to an oversaturation of carbon dioxide in the liquid phase. Carbon dioxide is therefore unavoidably stripped out. In a CO₂ electrolyzer, chemically and physically dissolved carbon dioxide always arises in the electrolyte, so that the described effects can rarely be completely avoided. They can, however, be minimized by means of suitable measures.

Suitable measures for this purpose are the compositions of the conductive salt that have already been described. In some embodiments, the proportion of hydrogencarbonate is as low as possible. This also applies to carbonates and hydroxides, since these are converted into hydrogencarbonate under the typical conditions in a CO₂ electrolyzer. It has been found that a maximum concentration of negative charge carriers in the conductive salt from a cumulative proportion of hydrogencarbonate ions, carbonate ions, hydroxide ions should be less than 20%, preferably less than 10%.

In addition, the operating pressure is as low as possible, since otherwise a significant proportion of carbon dioxide is physically dissolved in the electrolyte and thus gets into the anode region and is released again. 

What is claimed is:
 1. A CO2 electrolyzer comprising: a gas space adjoining a cathode; the cathode comprising a gas diffusion electrode adjoining a cathode space; an anode in an anode space; a membrane separating the cathode space from the anode space; wherein the cathode space accommodates a catholyte and the anode space accommodates an anolyte; a feed apparatus feeding reactant gas into the gas space; a mixing vessel for at least partial joint accommodation of the anolyte and the catholyte; wherein the mixing vessel includes a gas separating region closed off with respect to a surrounding atmosphere; and a connection line between the gas separating region and the gas space.
 2. The electrolyzer as claimed in claim 1, wherein: a mixture of the catholyte and the anolyte present in the mixing vessel has a total concentration of negative charge carriers; and the cumulative proportion of hydrogencarbonate ions, carbonate ions, and hydroxide ions is less than 20% of the total concentration of negative charge carriers.
 3. The electrolyzer as claimed in claim 1, further comprising a feed apparatus on the mixing vessel for the reactant gas; wherein the reactant gas is at least partially fed into the gas space via the mixing vessel.
 4. The CO2 electrolyzer as claimed in claim 1, wherein: a CO2-containing reactant gas is introduced into the gas space; after flowing through the cathode space or the anode space respectively, the catholyte and the anolyte are conducted into the mixing vessel; and a CO2-containing gas is evolved from a mixture of the liquid catholyte and the liquid anolyte and in turn is fed to the gas space as part of the reactant gas.
 5. The CO2 electrolyzer as claimed in claim 1, wherein an operating pressure prevailing in the electrolyzer is less than 5 bar.
 6. The CO2 electrolyzer as claimed in claim 1, wherein the reactant gas is conducted through the mixing vessel and introduced together with the CO2-containing gas evolved there into the gas space. 